The notion of functional interactions between the alpha 7 nicotinic acetylcholine ( alpha 7 nACh) and the cannabinoid systems is emerging from recent in vitro and in vivo studies. Both the alpha 7 nACh receptor and the cannabinoid receptor 1 (CB1) are highly expressed in the hippocampus. To begin addressing possible anatomical interactions between the alpha 7 nACh and the cannabinoid systems in the hippocampus, we investigated the distribution of neurons expressing alpha 7 nACh mRNA in relation to those containing CB1 mRNA. By in situ hybridization we found that the alpha 7 nACh mRNA is diffusely expressed in principal neurons and is highly expressed in a subset of interneurons. We observed that the pattern of distribution of hippocampal interneurons co-expressing transcripts encoding alpha 7 nACh and glutamate decarboxylase (GAD; synthesizing enzyme of gamma-aminobutyric acid, GABA) closely resembles the one displayed by interneurons expressing CB1 mRNA. By double in situ hybridization we established that the majority of hippocampal interneurons expressing alpha 7 nACh mRNA have high levels of CB1 mRNA. As CB1 interneurons contain cholecystokinin (CCK), we investigated the degree of cellular co-expression of alpha 7 nACh mRNA and CCK, and found that the cellular co-existence of alpha 7 nACh and CCK varies within the different layers of the hippocampus.[unreadable] In summary, we established that most of the hippocampal alpha 7 nACh expressing interneurons are endowed with the CB1 receptor. We found that these alpha 7 nACh/CB1 interneurons are the major subpopulation of hippocampal interneurons expressing CB1 mRNA. The alpha 7 nACh expressing interneurons represent half of the total population of CCK containing neurons in the hippocampus. Since it is well established that the vast majority of hippocampal interneurons expressing CB1 mRNA have serotonin type 3 (5-HT3) receptors, we conclude that these hippocampal alpha 7 nACh/ 5HT3/ CB1/CCK interneurons correspond to those previously postulated to relay inputs from diverse cortical and subcortical regions about emotional, motivational, and physiological states.